The present invention relates in general to waveform sampling system and more particularly to a waveform sampling system for detecting when a waveform being sampled has reached a predetermined signal level.
Oscilloscopes utilizing waveform sampling systems were developed more than twenty years ago to respond to small, fast-changing signals to which conventional oscilloscopes could not respond due to limited bandwidth or risetime characteristics. Sampling is a now well-known technique wherein a signal path is gated for an extremely short period of time to pass the substantially instantaneous amplitude value (voltage sample) of an electrical signal during that period. Each voltage sample taken in this manner is digitized and stored as magnitude data in a memory. In a sampling oscilloscope the magnitude data for each sample normally controls the vertical positioning display of a dot on a cathode ray tube screen, the horizontal position of the dot being set according to the sample timing. If enough waveform samples are taken at closely timed intervals, the resulting displayed dots can form an accurate representation of the waveform. Sampling systems are characterized by the sample timing method utilized. Sequential sampling systems sample a waveform at regular intervals and the dots representing sample data are plotted at regular horizontal locations to accurately represent the waveform. Random sampling systems sample a waveform at random times but the random sampling intervals are measured during sampling to produce sample timing data used to appropriately adjust the horizontal positioning of the dots representing the waveform.
Waveform sampling systems can be utilized to measure the rise time of an input waveform, "rise time" normally being defined as the time required for the waveform voltage to rise from 10% of its maximum level to 90% of its maximum level. One method of determining a rise time is to search all of the magnitude data in the sampling system memory after sampling a waveform to determine which sample points are closest to the 10% and 90% magnitude levels. The rise time is then equal to the interval between the sampling times associated with the 10% and 90% data. However in random and sequential sampling systems it is difficult to ensure that samples will be taken at a time when the waveform was acceptably close to the 10% or 90% magnitude levels. While the sampling frequency of sequential sampling systems of the prior art can be precisely controlled, the relative timing of sampling with respect to a triggering event (such as a zero crossing) in the waveform cannot be precisely controlled. Therefore in order to ensure that samples will be taken at times when the waveform magnitude is acceptably close to the 10% and 90% levels in either random or sequential sampling systems, it would appear the waveform should be sampled at a high rate during a period of interest. Since the size of memory required to store the sample data increases with the number of samples taken, there are practical limits to the accuracy which can be obtained by increasing the sampling rate.